On my 4000 I noticed the depth control lever would not move from the "D" position. Didn't think much of it and put it on a 3-point implement this week and it acted like it was set in "L". The control lever would have to be pulled way back and then all of a sudden there would be lift. Moving the lever forward to a point would result in sudden drop. Investigating I found that the load control lever on the rockshaft valve will not move at all. Is it possible that somebody's had the rockshaft control valve off and there is a cam, roller or lever inside in the wrong position? I have all the JD manuals and have studied them but have not had a control valve off yet so I'm just wondering what I'm looking for. Any ideas? Thank you.